Big mesh split

I am trying to separate the big Mesh into two, one for what is inside the yellow box and one for what is outside.
any idea how to get it?
meshboolean tools didn’t help me.
I tried to upload the file but it was too heavy and it would not let me

Hi Ivo -

Which version of Rhino are you using?
Have you checked the input mesh with MeshRepair?

You can use Rhino - Upload to Support to send that to us so that we can have a look. Make sure to mention this thread in the comments field.

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as it breaks when I make a trim, I need a clean cut, splitting the mesh in two, whit the help of the yellow box

try this in the v8 wip… the new mesh splitting and mesh booleans work way better than v7 ever did (or will)

Here is the Mesh Repair report in Rhino 8

Here is what is wrong with this mesh:
Mesh has 8583 non manifold edges.
Mesh has 8 duplicate faces.
Skipping face direction check because of positive non manifold edge count.
Mesh has 14333 pairs of faces that intersect each other.
This can cause problems if you’re doing mesh boolean operations with it.

The site mesh is a real mess of a mesh. Much of the problem is not understanding how to put the mesh back together after doing the split. If the Split disjoint mesh is run the 3d map is 8716 separate meshes that overlap and or have gaps around each edge. Each tile color here is a different mesh that does not line up with its neighbor. It is not clear how to get those edges to create a mesh that is valid.

Technically we could try to intersect all 8000 meshes with the box. Then see what is possible.

Is there any way to get a cleaner mesh?

perhaps you should think about redoing the mesh due to its complexity.